1. Field of the Invention
The present invention relates to a process for forming a monocrystalline thin film of silicon, which is suitable for formation of monocrystal growth layers of the element semiconductor of silicon with precision as precise as a single monolayer.
2. Discussion of related art
A chemical vapor deposition process (referred to hereinafter as a CVD process) and a molecular beam epitaxy (referred to hereinafter as an MBE process) are well known in the art as vapor phase epitaxial techniques for forming a crystalline thin film of an element semiconductor consisting of a single element such as silicon. According to the CVD process, a silicon compound, which is a source, and gas such as hydrogen gas, which is a carrier, are simultaneously introduced into a reaction chamber to cause growth of a crystal by means of thermal decomposition. However, the thermal decomposition results in a poor quality of the crystal layer formed by growth. The CVD process is also defective in that difficulty is encountered for controlling the thickness of the layer with precision as precise as a single molecular layer.
On the other hand, the MBE process is well known as a crystal growth process making use of a ultrahigh vacuum. This process, however, includes physical adsorption as its first step. Therefore, the quality of the crystal is inferior to that provided by the CVD process which makes use of a chemical reaction. Further, due to the fact that the sources themselves are disposed in a growth chamber, it is difficult to control the amount of gases produced by heating the sources, to control the rate of vaporization of the sources and to replenish the sources, resulting in difficulty of maintaining a constant growth rate for a long period of time. Further, the evacuating device discharging, for example, the vaporized matters becomes complex in construction. Furthermore, it is difficult to precisely control the stoichiometric composition of the compound semiconductor. Consequently, the MBE process is defective in that a crystal of high quality cannot be obtained.
In the MBE process, individual component elements of a compound semiconductor are simultaneously deposited by vacuum evaporation. An atomic layer epitaxial process (referred to hereinafter as an ALE process) is an improvement over the MBE process. This ALE process is featured by alternately depositing individual component elements of a compound semiconductor, as disclosed in U.S. Pat. No. 4,058,430 (1977) to T. Suntola et al. and also in J. Vac. Sci. Technol. A2, (1984), page 418 by M. Pessa et al. Although the ALE process is suitable for the growth of a I-VII compound, a II-VI compound, a III-V compound or an oxide of such elements, an excellent crystalline property cannot be expected inasmuch as the ALE process is an extension of the MBE process. Rather, the ALE process is suitable for the growth of a crystal on a substrate of glass, and it is difficult with the ALE process to achieve selective epitaxial growth of a crystal which is important in the field of production of semiconductor integrated circuits and the like. An attempt has been made to attain crystal growth by the ALE process utilizing a chemical reaction instead of resorting to the ALE process utilizing the vacuum evaporation. Although the attempt has succeeded in the formation of a polycrystalline II-VI compound such as ZnS or an amorphous compound such as Ta.sub.2 O.sub.5, it has not been successful for the growth of a single crystal. As described in U.S. Pat. No. 4,058,430 (1977), the ALE process is based on the principle of depositing a monomolecular layer of one of component elements of a compound on a monomolecular layer of another component element of the compound. Therefore, the ALE process is limited to the growth of a thin film of a compound and is not applicable to the growth of an element semiconductor such as Si or Ge. On the other hand, one of the inventors has reported, in a magazine entitled "Electronic Materials", December 1981, page 19, as to the possibility of application of a developed version of the ALE process to the growth of a single crystal of Si. However, the paper does not teach any practical information of the factors including the growth temperature and gas introduction rate.
Thus, with the CVD process and MBE process, it is difficult to form a high-quality crystal with precision as precise as a single molecular layer. With the ALE process, a single crystal cannot be formed by growth, and, especially, growth of an element semiconductor such as Si or Ge is impossible in principle.
U.S. Pat. No. 4,834,831, whose contents are incorporated herein by reference, discloses a method for growing a single crystal thin film of an element semiconductor. Under the growth chamber environmental conditions employed, the results show that the film thickness grown per cycle varies continuously in response to variations in substrate temperature or feeding pressure from a half molecular layer (0.68 Angstroms) to a monomolecular layer (1.36 Angstroms) to a dimolecular layer (2.72 Angstroms). Due to this continuous variation in growth of the molecular layer per cycle in response to variations in temperature or pressure, the environmental conditions within the growth chamber require precise control.
A research paper entitled "PROJECT FOR PROMOTING DEVELOPMENT OF CREATIVE SCIENCE & TECHNOLOGY, NISHIZAWA PROJECT ON PERFECT CRYSTALS, COLLECTIVE SUMMARIES OF RESEARCHES", Research Development corporation of Japan, dated Dec. 10, 1986, discloses in section 2.6 the molecular layer epitaxy of Si by means of a SiH.sub.2 Cl.sub.2 system in which SiH.sub.2 Cl.sub.2 and H.sub.2 are alternately introduced as raw gases into a crystal growth chamber and onto a substrate of Si heated in a vacuum. Although the paper fails to discuss the selection criteria used for deciding upon appropriate values of temperature, pressure, duration of gaseous compound introduction or duration of vacuum within the growth chamber that were used in the experiment, it does mention some results.
This research paper mentions that when H.sub.2 is used with its pressure greater or equal to 3.0.times.10.sup.-5 Torr, the film growth thickness per cycle tends to saturate for the pressure of SiH.sub.2 Cl.sub.2 of from 2.0.times.10.sup.-4 to 7.0.times.10.sup.-4 Torr. The saturation thickness of the film growth corresponds to the thickness of a single atomic layer. Further, the film growth thickness per cycle shows a steep rise over the thickness of a single atomic layer when the pressure of SiH.sub.2 Cl.sub.2 is increased over the value of 7.0.times.10.sup.-4 Torr. When the pressure of SiH.sub.2 Cl.sub.2 is less than or equal to 1.0.times.10.sup.-4 Torr, the film growth thickness per cycle is determined by the pressure of SiH.sub.2 Cl.sub.2 irrespective of the pressure of H.sub.2.
Also, the substrate temperature dependence of the film growth thickness per cycle was investigated. The film growth per cycle shows a steep rise near and above a temperature of 890 degrees Celsius; between 825 to 890 degrees Celsius the film growth was fairly constant at just beneath about one Angstrom.
It would be desirable to exploit the experimental results mentioned in this research paper commercially to achieve semiconductor thin film growth as precise as a monolayer.